1. Field of the Invention
The present invention relates to an optical information reproducing apparatus for reproducing information from an optical recording medium, such as an optical disk or an optical card, in which the information is stored in a state that it is optically reproduced. More particularly, the invention relates to an optical information reproducing apparatus for reproducing information from an information recording medium in which information is recorded in a multi-layer fashion.
2. Discussion of the Related Art
In general optical recording media, such as optical disks, store information in a two-dimensional plane. Constant pressure to increase the record capacity of the recording medium has led to another information recording approach in which information is three-dimensionally recorded. Haslhimoto, Kawada, and Kawada describe an example in which information is three-dimensionally recorded in a photo polymer in which the refractive index varies with light received, in their paper "Co-focal Point 3-Dimensional Photo Refractive Optical Memory", Optics Joint Symposium Kyoto, '92, BaO3 (1992).
FIG. 28 is a diagram schematically showing a conventional information reproducing apparatus for reading information out of an optical multi-layer recording medium in which the information is three-dimensionally stored. An optical multi-layer recording medium 94 is a bulky photo polymer in which information is three-dimensionally stored. The refractive index of record pits 95a and 95b is different from that of the surrounding polymer. Because of this, light reflection takes place at the interface between each of the record pits 95a and 95b and its surrounding polymer. In the reproducing apparatus of FIG. 28, a light beam reflected at the interface is read by a scanning optical microscope of the cofocal point type. The laser beam emitted from a laser light source 91 goes through a half-mirror 92 and a lens 93, and is focused at a record pit 95a. The laser beam reflected at the position of the record pit 95a goes through the lens 93, the half-mirror 92 and an aperture 96, and reaches a photo sensing device 97. The record pit 95a is optically conjugate with the aperture 96. The reflected light beams from the record pit 95b located above the record pit 95a are not focused at the aperture 96. Accordingly, the recorded information can be separated to some extent in the direction of depth. The optical multi-layer recording medium 94 is three-dimensionally movable by a three-dimensional stage 98.
In the conventional apparatus shown in FIG. 28, the difference of refractive index between each record pit and its surrounding polymer causes light beam reflection. Therefore, in making an access to a lower recording layer, a probe light beam is inevitably attenuated until it reaches the lower recording layer. This results in an insufficient S/N ratio in the depth direction.